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    Need help determining correct bias adjustment

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    notboating

    Posts : 11
    Join date : 2013-12-17
    Location : Western PA

    Need help determining correct bias adjustment

    Post by notboating on Sun Sep 21, 2014 8:32 pm

    I have a pair of modified Mk III's. How do I go about determining what the correct bias voltage is? Have not had any success locating a hi-fi shop in the Pittsburgh area that I can go to for service. Want to check/adjust bias but am hesitant as the amps are not in their original configuration.

    Thanks for your help!

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Need help determining correct bias adjustment

    Post by Bob Latino on Mon Sep 22, 2014 9:39 am

    Hi,

    See if you can post a photo of the inside wiring so we can look at how your Mark III's are wired. If you have stock Mark III amps, they have a single 11.2 ohm bias resistor. I would rather not give any bias information until we can look at the amp's internal wiring to make sure that your amps don't have some other non-stock wiring.

    Note - See the sticky above on how to post a photo on the Dynaco Tube Audio Forum ..

    Bob

    notboating

    Posts : 11
    Join date : 2013-12-17
    Location : Western PA

    Re: Need help determining correct bias adjustment

    Post by notboating on Mon Sep 22, 2014 7:46 pm

    Thanks. here is the bottom of 1 of them.



    Here is a link to the pictures of the Mk iii's on picasaweb

    http://tinyurl.com/ljebpp7

    Please let me know if you can see them or not so I can try something else if this does not work.

    DynakitParts
    Admin

    Posts : 183
    Join date : 2008-11-30

    MK III

    Post by DynakitParts on Mon Sep 22, 2014 10:12 pm

    Looks stock to me...just a few newer board components. Still has the 11.2 ohm bias resistor and 10K pot.

    I would just set the bias to about 1.35 vdc and see how it sounds....Factory suggested setting is 1.56 vdc.

    Kevin

    notboating

    Posts : 11
    Join date : 2013-12-17
    Location : Western PA

    Re: Need help determining correct bias adjustment

    Post by notboating on Tue Sep 23, 2014 6:12 am

    Thanks. If factory suggested is 1.56 then why do you recommend 1.35?

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Need help determining correct bias adjustment

    Post by Bob Latino on Tue Sep 23, 2014 8:07 am

    IMHO Kevin is correct in recommending a more modest bias setting on older original Mark III's. If a 1.56 VDC bias setting is used then each (KT88 or 6550) output tube will have a bias current of about 70 milliamps which is fairly high by today's standards. A bias setting of 1.35 volts on your Mark III's will give a bias current of about 60 milliamps  on each output tube. The output tubes will sound the same and give better tube life if you use 1.35 VDC as your bias setting.

    Bob

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Need help determining correct bias adjustment

    Post by zx on Tue Sep 23, 2014 9:20 am

    Nice amps....i have 3pr of the stock MK3s.... what is your B+ ......if like mine it high over 500 i run my MK3s on a Varic.....an drop the B+ to about 480-90 for the best sound...then bias the Amps...
    In the 50-60s when these amps were made the wall ACV was lower...like 117....today most are 125 are higher....so this push up your B+....Not good on the parts....



    Thanks for the site Bob.................

    notboating

    Posts : 11
    Join date : 2013-12-17
    Location : Western PA

    Re: Need help determining correct bias adjustment

    Post by notboating on Tue Sep 23, 2014 6:56 pm

    zx, no idea what you are referring to. How about some detailed instruction?

    tubes4hifi
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    Re: Need help determining correct bias adjustment

    Post by tubes4hifi on Sat Sep 27, 2014 1:09 am

    hopefully since you have a tube amp you should have a voltmeter (or DVM) and you can check the AC wall socket to see about how much AC voltage you have. If around 115-120v you are OK, but if closer to 125vac you may want to invest about $60 in a variac, which will allow you to adjust the voltage down and it's easier on the amp. You'll also need the voltmeter to set the correct bias on the tubes.

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Need help determining correct bias adjustment

    Post by zx on Sat Sep 27, 2014 11:56 am

    Did not mean to spek Voodoo to you....


    ...Some times less is more..if your MK3s amps sound good... that all that matters...by the way how do thay sound??
    This old guy some times acks like.... i need too get these Dynaco amps to last for ever....mine have over 50years on them now.... an thay are rocking.. with some mods...an the right over all B+
    When tube amps were made in the 50-60s the AC comeing in from the wall could as low as 110-15....so the MK3s B+ at that ACv 470-480 B+....
    To day it higher...if you have 120-125 B+ can go as high 550 .... some of the parts are rated at lower 500-525...This can stress the parts....an as Bob has said you need to drope the bias.....
    but with the Varac i can set the wall AC back to 110-115...are what ever lower AC......to me tube amps sound better at the lower B+...480 b+ is about the top for me ......other well say







    ...it all about the sound.



    thanks for the site Bob..................

    notboating

    Posts : 11
    Join date : 2013-12-17
    Location : Western PA

    Re: Need help determining correct bias adjustment

    Post by notboating on Mon Sep 29, 2014 5:55 am

    Do you have a recommendation in what to look for in a variac? Can a single variac support 2 mk iii's ?

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Need help determining correct bias adjustment

    Post by zx on Mon Sep 29, 2014 9:09 am

    Yes............
    I run two MK3 all the time....that way i know there each geting the same V.... 5 amps Variac fine...
    i got most of minne at fle-market yard sals for $10-20....but $40-50 for a new one...
    I do repairs on tube amps for others....an if i sale any tube preamps are amps over 40 years old..
    thay get a Variac....in the sale...
    I see a lot of my altime fav sounding tube amps an pres....hhshott...have all there parts socalled upgreaded....to last longer...an sound better....
    then soon after most of theses are up for sale....
    i have found some of hhscott old parts have a sweetness...that make the sound... i an others have not made better with new-better parts............
    But the high AC coming in wall today can kill the old parts ...so the Variac saves the day for most of all the old parts i have found....an yes this is just one-mans o-pine.....
    Now i would not say that SS amps should be ran for a long time with a Variac....thay pull a lot of Amps...
    tubes pull mil-Amps.....others can say there thinking





    thanks for the site Bob...........................

    notboating

    Posts : 11
    Join date : 2013-12-17
    Location : Western PA

    Re: Need help determining correct bias adjustment

    Post by notboating on Mon Sep 29, 2014 2:16 pm

    what current rating should i be looking for in a variac that would cover a pair of mk-iii's up to their power rating with some buffer....

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: Need help determining correct bias adjustment

    Post by tubes4hifi on Mon Sep 29, 2014 3:29 pm

    for two power amps I'd recommend a 5A variac, there are some here:

    http://www.parts-express.com/Search.aspx?keyword=VARIAC%20TRANSFORMER&sitesearch=true

    Bob Latino
    Admin

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    Re: Need help determining correct bias adjustment

    Post by Bob Latino on Mon Sep 29, 2014 4:52 pm

    For a 5 amp variac, I had a customer point out this one at the link below on Ebay .. It's made in China but for $50 + shipping, it seems like a good buy especially if you live in an area that has voltages much above 120 VAC. Everyone should check their line voltage from time to time. If your voltage is above 122 VAC, you should probably get one, set it to 117 - 118 VAC, and run your tube amp off the variac.

    Generico variac on Ebay

    Bob

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Need help determining correct bias adjustment

    Post by peterh on Mon Sep 29, 2014 5:10 pm

    Bob,
    as you control the transformers, why don't you create some models that has extra primaries ?

    Having just a few ( 5V 10V 20V 115V ) and a patching are one could connect and combine
    windings to suit.

    This is what a lot of stuff had in europe in the 50-60, just look at a phillips amp and
    see what possibilyties they had. Some even had special "voltage selectors" where
    the user could easily turn a "plug" to the wanted voltage.

    Ask your transformer builder for a pricequote. Maybe your kits could benefit for this ?
    The total cost for an owner could be less then having to get a variac.

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